A conventionally known variable valve timing mechanism disclosed, for instance, in Patent Documents 1 to 6 changes the valve timing of an internal combustion engine by changing the phase angle of a camshaft relative to a crankshaft. A common variable valve timing mechanism includes a housing, which is coupled to the crankshaft by a belt or chain, and a vane assembly, which is fastened to the camshaft. The vane assembly is contained in the housing to form a hydraulic chamber between the vane assembly and the housing. When oil is supplied to the hydraulic chamber with the amount of oil supply controlled by a control valve, the vane assembly and the housing can be relatively rotated to change the phase angle of the camshaft relative to the crankshaft.
When the above hydraulic variable valve timing mechanism is used, oil viscosity affects its operating characteristics. For example, the higher the oil viscosity, the lower the responsiveness of the variable valve timing mechanism. Meanwhile, the lower the oil viscosity, the larger the amount of oil leakage from a clearance between sliding parts. Therefore, when the variable valve timing mechanism is to be controlled with increased accuracy, it is preferred that oil viscosity be reflected in the control of the variable valve timing mechanism. However, an existing system does not have a viscosity sensor that directly measures the oil viscosity. Therefore, it is necessary to install a viscosity sensor or add some means for indirectly viscosity measurement. It goes without saying that adding a viscosity sensor increases the manufacturing cost. Further, an increase in the number of sensors will increase the probability of failure.
Some of the disclosures in Patent Documents 1 to 6 take the oil temperature into account when controlling the variable valve timing mechanism. The oil viscosity is temperature-dependent and varies with oil temperature. More specifically, the oil viscosity is high at a low temperature and decreases with an increase in the oil temperature. Therefore, when the variable valve timing mechanism is controlled in consideration of oil temperature, the oil viscosity is indirectly reflected in the control of the variable valve timing mechanism.
However, the temperature dependence of oil viscosity varies with the degree of oil deterioration. More specifically, the viscosity of deteriorated oil is high whereas the viscosity of fresh oil is low even when the oil temperature remains unchanged. Further, the viscosity of fresh oil varies with its composition. Therefore, the current oil viscosity cannot be accurately estimated simply by considering the oil temperature.
The disclosure in Patent Document 2, on the other hand, focuses attention on the influence of oil viscosity on the responsiveness of the variable valve timing mechanism, and determines the viscosity in accordance with the responsiveness. More specifically, a response speed-viscosity map is prepared to define the relationship between the viscosity and the response speed of the variable valve timing mechanism and used to calculate the viscosity. The response speed-viscosity map is prepared by first determining the relationship between the oil temperature and the viscosity of fresh oil, then determining the relationship between the oil temperature and the response speed of the variable valve timing mechanism, and incorporating these two relationships into a single whole. According to this response speed-viscosity map, the current oil viscosity can be accurately determined by operating the variable valve timing mechanism and calculating its response speed.    [Patent Document 1] JP-A-2002-349300    [Patent Document 2] JP-A-2004-92593    [Patent Document 3] JP-A-2001-164953    [Patent Document 4] JP-A-11-141359    [Patent Document 5] JP-A-11-210424    [Patent Document 6] JP-A-2006-77680